This invention relates to a process for monitoring the tightness of packs disposed in a measuring chamber. The invention also relates to an arrangement for carrying out this process.
Arrangements for the non-destructive testing of flexible packs for leaks are already known. German Utility Model G 8 128 651 describes one such arrangement comprising a measuring chamber designed to receive the sample to be tested and a feeler in said chamber which is sensitive by contact to variations in pressure and which is connected to a calculator for recording the variations in pressure as a function of time. The disadvantages of an arrangement such as this are that the measuring cells are disposed in the measuring chamber which necessitates careful handling in view of the fragility and sensitivity of the measuring cells. On the other hand, since the feeler is only sensitive by contact, high sensitivity of measurement cannot be expected in view of the risks of unevenness of the sample to be measured. Finally, this known arrangement can only be used for testing flexible packs.
Other arrangements for detecting leaks in rigid objects or packs are also known. The system according to U.S. Pat. No. 2,467,767 comprises a measuring chamber which operates under excess pressure and which is equipped with liquid manometers. The entire system is manually controlled without any automation. All the pressure values have to be acquired by successive observations on the part of the operator who deduces the nature of any leaks therefrom by comparison. The principal limitation of this known process in regard to microleaks is that it is difficult to evaluate the small difference between the values successively read off from the manometer connected to the test chamber, the range of measurement of this manometer being at least equal to the total pressure of the fluid contained in the chamber. As a result, the reliability of the measurements depends upon human parameters, i.e., upon the quality of the observations of the operator. Finally, U.S. Pat. No. 2,467,767 does not make any reference to tests under reduced pressure and is not designed to be used for the detection of leaks in non-rigid objects or packs.
U.S. Pat. No. 3,331,237 describes a system having improved sensitivity through the use of a differential manometer. Unfortunately, this known system is only applicable to solid objects comprising an internal cavity capable of being connected to the measuring equipment by a connecting hose. Accordingly, it cannot be applied either to packs, such as flexible bags, or to rigid packs containing products, connection without destruction of the pack being impossible.
U.S. Pat. No. 3,504,528 describes a system which is also based on differential measurement, the comparison being made between two chambers, namely a measuring chamber and a reference chamber. These two chambers are symmetrically inserted into two circuits connected in parallel from a first chamber initially charged with a gas under pressure. This known system is only applicable to solid objects, such as non-deformable shell cases, and the test can only be carried out under excess pressure. Another disadvantage of this known system lies in the permanent connection of the differential sensor to the two chambers which can lead to overloading of the allegedly very sensitive sensor unless the valves are perfectly synchronized or in the event of a sudden leak in one of the chambers.
Publication Wo 81/01 333 claims an arrangement which is based on the transfer of predetermined quantities of a gas, the differential measurement of the pressure being effected by comparison between two branches which are symmetrical in volume or at least proportional to one another. Intended according to the author for monitoring objects of defined shape comprising a cavity it is desired to test for leaks, this known arrangement is not designed to be applied to flexible bags containing a product, for example, a powder, of which the shape and the volume vary according to the pressure applied and the quantity of gas contained in the bag, these variations being capable of reaching significant proportions without common measurement with those emanating from the manufacturing tolerances of a solid object to the shape of which the measuring chamber is adapted. In addition, this known arrangement has the disadvantage of necessitating the modification of at least two volumes or chambers in the event of a change in the dimensions of the object to be tested.
In addition, the pressures in the system depend upon the feed pressure, for which no regulating or measuring arrangement is provided, so that the detection of a major leak in the described process is rendered uncertain.